1. Field
This disclosure relates generally to reducing overhead in a wireless communication system and, more specifically, to techniques for reducing preceding overhead in a multiple-input multiple-output wireless communication system.
2. Related Art
Today, multiple-input multiple-output (MIMO) systems, which employ multiple antennas at a transmitter and multiple antennas at a single receiver or one or more antennas at multiple receivers (depending on the implementation), are becoming increasingly common. Single-user MIMO systems implement multiple antennas at a transmitter and multiple antennas at a receiver. In contrast, multi-user MIMO systems employ multiple antennas at a transmitter and consider multiple receivers (each of which may have one or more antennas) as spatial resources, with each of the multiple receivers corresponding to at least one output. In general, MIMO wireless communication systems exhibit increased data throughput (due to higher spectral efficiency) and increased link range (due to reduced fading) without requiring additional bandwidth or transmit power, respectively (as contrasted with multiple-input single-output (MISO), single-input multiple-output (SIMO), and single-input single-output (SISO) wireless communication systems). MIMO wireless communication systems generally employ precoding, spatial multiplexing (SM), diversity coding, a combination of SM and precoding, or a combination of SM and diversity coding.
In SM (which can be employed with or without channel state information (CSI) at a transmitter), an original signal is split into multiple lower-rate streams and each stream is transmitted from a different transmit antenna in the same frequency band. When the transmitted streams arrive at a receiver antenna array with sufficiently different spatial signatures, a MIMO receiver can readily separate the transmitted streams into parallel channels that exhibit increased signal-to-noise ratio (SNR), as compared to the original signal when transmitted as a single higher-rate stream.
Precoding employs beamforming to support multi-layer communications. Precoding normally utilizes knowledge of CSI at a transmitter in an attempt to maximize received signal levels at all antennas of a receiver. Precoding can be generally defined as a transformation applied to the transmitted data before the transmission, typically to align the transmission to the channel in some form to maximize a performance metric like signal-to-noise ratio (SNR). Precoding, in general, can be a linear or non-linear transformation. In linear preceding, the transformation can be equivalently applied in the form of a matrix to the transmitted vector symbol on the multiple antennas. Typically, some form of channel knowledge is used at the transmitter to choose an appropriate precoder. In some cases, a receiver feeds back information about the channel or the precoder to a transmitter. Codebook based preceding is a special case of precoding and, in some cases, a preferred or recommended precoder is chosen from a set of known precoders (known as a codebook) with each precoder in the set being associated with an index, which is fed back to the transmitter as a means for feedback reduction. In general, precoding can be applied to transmit single or multiple MIMO streams to a single user (single-user MIMO) or multiple users (multi-user MIMO).
MIMO wireless communication systems are known that have fed-forward (or contemplated feeding-forward) preceding information in the form of control bits or precoded pilot signals from a BS to an SS to explicitly signal to the SS what precoder the BS employed in a MIMO transmission.
Precoding may be employed in orthogonal frequency division multiplexing (OFDM) systems, which typically support relatively high data rate wireless transmission using orthogonal channels that offer immunity against fading and inter-symbol interference (ISI) without requiring implementation of elaborate equalization techniques. Typically, OFDM systems split data into N streams, which are independently modulated on parallel spaced subcarrier frequencies or tones. The frequency separation between subcarriers is 1/T, where T is the OFDM symbol time duration. Each symbol may include a guard interval (or cyclic prefix) to maintain the orthogonality of the symbols. Normally, OFDM systems have utilized an inverse discrete Fourier transform (IDFT) to generate a sampled (or discrete) composite time-domain signal.
Various wireless networks, such as third-generation partnership project long-term evolution (3GPP-LTE) and IEEE 802.16 (also known as worldwide interoperability for microwave access (WIMAX)) compliant architectures employ a scheduler (included within or coupled to a serving base station (BS)) that utilizes information derived from channel characterization to determine channel allocation for served user equipment (subscriber stations (SSs)). In a 3GPP-LTE compliant system, channel allocation, e.g., uplink and downlink assignments, is provided to SSs over a downlink shared control channel (physical downlink control channel (PDCCH)), which typically includes one or more control channel symbols. The one or more control channel symbols are usually transmitted by a serving BS at a beginning of a downlink subframe. Typically, upon receiving the one or more control channels symbols, each of the SSs searches (using, for example, a blind search procedure) the one or more control channel symbols to locate an associated downlink and uplink control channel to determine respective downlink and uplink assignments.
With reference to FIG. 1, a functional block diagram 100 of a relevant portion of a conventional multiple-input multiple-output (MIMO) wireless communication system 100, which may be 3GPP-LTE compliant, is depicted. The system 100, which may be configured to operate in a single-user MIMO mode or a multi-user MIMO mode, includes a base station (BS) 102 that is in communication with a subscriber station (SS) 142. As is depicted, the BS 102 includes a scheduler 104, a BS precoder (preceding codeword (CW)) codebook (CB) 106, a precoder selector 108, a control field generator 110, and a transmitter 112. The BS 102 may also employ an SS CB 116 that is coupled to the scheduler 104, when the BS precoder CB 106 is not the same as SS CB 152 (included in the SS 142). As is logically shown, the scheduler 104 receives feedback (FB) from the SS 142 that corresponds to a recommended precoder (preceding CW) selected by the SS 142. The FB from the SS 142 may correspond to a CB index, e.g., an SS CB index or a channel vector quantizer (CVQ) CB index.
When the FB corresponds to a CB index, the scheduler 104 utilizes the CB index (unless the scheduler 104 decides to override the recommended precoder) to select an appropriate precoder from the BS precoder CB 106. The selected precoder is passed by the scheduler 104 to the precoder selector 108, which provides a CB index for the selected precoder to the control field generator 110 and the selected precoder to the transmitter 112, which includes multiple antennas 114. The control field generator 110 positions the CB index in a protocol dependent field of, for example, an associated downlink control channel of the SS 142 (which is included in a downlink shared control channel) or in another location in the downlink shared control channel to feed-forward (FF) the selected CB index to the SS 142.
The SS 142 includes a control channel decoder 144, a preceding CW decoder 146, a receiver 148, a channel estimator 150, the SS CB 152 (which may include a channel vector quantizer (CVQ) CB or a precoder CB), and a precoder selector 154. The SS 142 may also employ a BS CB 158, which is coupled to the precoding CW decoder 146, when the BS precoder 106 is not the same as the SS CB 152. The control channel decoder 144, for example, decodes an associated downlink control channel of the SS 142 to retrieve a CB index (which is fed-forward from the BS 102 in the downlink shared control channel) and provides the CB index to the precoding CW decoder 146, which provides a single CW (i.e., ‘W’) to the receiver 148 based on the CB index, irrespective of whether the SS CB 152 is a precoder CB or a CVQ CB. The receiver 148, which is coupled to one or more antennas 156, utilizes the preceding CW to process a received signal. As is shown, the receiver 148 is coupled to the channel estimator 150, which estimates a channel quality of a received signal and provides a channel estimation (i.e., ‘H’) to the precoder selector 154, which selects a recommended precoder (precoding CW) or a CVQ from the SS CB 152 responsive thereto. The precoder selector 154 also provides the FB (which may be in the form of a CB index) to the scheduler 104 (of the BS 102) in a protocol dependent location in an assigned uplink (JL) to indicate the recommended precoder.
With reference to FIG. 2, a flowchart of a conventional process 200, which feeds-forward precoding information to the SS 142 and is employed in the BS 102 when the system 100 is configured in single-user MIMO mode, is depicted. In block 202 the process 200 is initiated, at which point control transfers to block 204 where the BS 102 receives feedback (FB), which corresponds to a CB index of a recommended precoder, from the SS 142. Next, in decision block 206, the BS 102 makes a determination at to whether the recommended precoder should be overridden. When the BS 102 decides to override the FB in block 206, control transfers to block 208 where a non-recommended precoder is selected (from the BS precoder CB 106) by the BS 102 for a transmission. In block 206, when the BS 102 decides not to override the FB, control transfers to block 210 where the recommended precoder is selected for the transmission. Following blocks 208 and 210, control transfers to block 212 where precoding information (e.g., in the form of control bits or precoded pilot signals) are fed-forward in the transmission from the BS 102 to the SS 142. Following block 212, control transfers to block 214 where control returns to a calling routine.
With reference to FIG. 3, a flowchart of another conventional process 300, which feeds-forward preceding information to the SS 142 and is employed in the BS 102 when the system 100 is configured in multi-user MIMO mode, is depicted. In block 302 the process 300 is initiated, at which point control transfers to block 304 where the BS 102 receives feedback (FB), which corresponds to a CVQ CB index, from the SS 142. Next, in decision block 306, the BS 102 determines whether to override the FB provided by the SS 142. When the BS 102 decides to override the FB, control transfers from block 306 to block 308. In block 308, a non-recommended precoder is selected (from the BS precoder CB 106) by the BS 102 for a transmission. In block 306, when the BS 102 decides not to override the FB, control transfers to block 310 where the BS 102 uses the CVQ CB index to choose a precoder from a subset of precoders (which may include one or more precoders depending on the system configuration) indicated by the CVQ CB index for the transmission. Following blocks 308 and 310, control transfers to block 312 where preceding information (e.g., in the form of precoded control bits or precoded pilot signals) is fed-forward in the transmission from the BS 102 to the SS 142. Following block 312, control transfers to block 314 where control returns to a calling routine.
With reference to FIG. 4, a flowchart of yet another conventional process 400, which receives fed-forward preceding information from the BS 102 and is employed in the SS 142 when the system 100 is configured in single-user or multi-user MIMO mode, is depicted. In block 402, the process 400 is initiated, at which point control transfers to block 404, where the SS 142 receives a transmission from the BS 102. Next, in block 406, the SS 142 identifies a precoder associated with the transmission from a CB based on the CB index included in the transmission. Then, in block 408, the transmission is decoded with the identified precoder. Following block 408, control transfers to block 410 where control returns to a calling routine.
In the above-described techniques, precoding information (i.e., a CB index for a preceding codeword) for a multiple input multiple output (MIMO) transmission is explicitly fed-forward (FF) from a serving BS to an SS to advise the SS as to the precoder that was utilized for the transmission. In this case, the precoding information is compared with preceding information recommended by the SS to determine whether a recommended precoder was utilized by the SS. While the approach provides certainty as to what precoding CW a serving BS has employed in preceding a transmission, feeding forward the precoding information from the BS to the SS is not without cost, as a number of bits (e.g., three to five bits depending on the number of SSs assigned to a resource block (RB)) may be employed for each MIMO RB allocated. While the number of bits may be reduced to, for example, one bit (which may be used to indicate whether the recommended precoder was utilized or not for the RB) for each MIMO RB, feeding-forward one bit for each MIMO RB may still result in unacceptable preceding overhead when the number of MIMO RBs is relatively large.
What is needed are techniques for reducing preceding overhead in a multiple-input multiple-output wireless communication system.